A qtree is a logically defined file system that can exist as a special subdirectory of the root directory within an internal volume. You can create up to 4,995 qtrees per internal volume. There is no maximum for the storage system as a whole.
In general, qtrees are similar to internal volumes. However, they have the following key differences:
a) Snapshot copies can be enabled or disabled for individual internal volumes but not for individual qtrees. b) Qtrees do not support space reservations or space guarantees. c) There are no restrictions on how much disk space can be used by the qtree or how many files can exist in the qtree.
A traditional volume is a volume that is contained by a single, dedicated, aggregate. It is tightly coupled with its containing aggregate. No other volumes can get their storage from this containing aggregate.
The only way to increase the size of a traditional volume is to add entire disks to its containing aggregate. You cannot decrease the size of a traditional volume. The smallest possible traditional volume uses all the space on two disks (for RAID4) or three disks (for RAID-DP).
Note: All volumes created with a version of Data ONTAP earlier than 7.0 are traditional volumes. If you upgrade to Data ONTAP 7.0 or later, your volumes and data remain unchanged, and the commands you used to manage your volumes and data are still supported.
A FlexVol volume is a volume that is loosely coupled to its containing aggregate. A FlexVol volume can share its containing aggregate with other FlexVol volumes. Thus, a single aggregate can be the shared source of all the storage used by all the FlexVol volumes contained by that aggregate.
Because a FlexVol volume is managed separately from the aggregate, you can create small FlexVol volumes (20 MB or larger), and you can increase or decrease the size of FlexVol volumes in increments as small as 4 KB.
You can also refer the below link which talks about, Data ONTAP 8: FlexVol Volumes FAQ